workflow sends a pdf thru outlook but users cannot open

I have a workflow that sends an email to users and attaches a pdf form of a document.  this works ok except the end users cannot open the pdf.  the error message says the pdf has not been decoded.  I've looked around online and it appears to be something with the sharepoint / pdf emailing.  I've set my exchange server to send all attachments in MIME but still doesn't work.  does anyone have other ideas to try?  it's necessary that the file is sent pdf so it cannot be modified.